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Unconsolidated Aquifer Systems of Posey County, Indiana

by

Gerald A. Unterreiner

Division of Water, Resource Assessment Section

July 2006

Nine unconsolidated aquifer systems have been mapped in Posey County: the Unglaciated

Southern Hills and Lowlands; the Dissected Till and Residuum; the Alluvial, Lacustrine, and

Backwater Deposits; the Buried Valley; the Wabash Lowland Till Subsystem; the Wabash River

and Tributaries Outwash; the Ohio River Outwash; the Wabash River and Tributaries Outwash

Subsystem; and the Ohio River Outwash Subsystem. Boundaries of these aquifer systems are

commonly gradational and individual aquifers may extend across aquifer system boundaries.

Sand and gravel were deposited of sufficient thickness and extent in places within the main

valleys of the Wabash and Ohio Rivers to make up an aquifer capable of supplying large

municipal, industrial, and irrigation needs. Outside of the main valleys of the Wabash and Ohio

Rivers, much of the county is covered by relatively low permeable material overlying the

bedrock, and ground-water resources from unconsolidated deposits are minimal.

Regional estimates of aquifer susceptibility to contamination from the surface can differ

considerably from local reality. Variations within geologic environments can cause variation in

susceptibility to surface contamination. In addition, man-made structures such as poorly

constructed water wells, unplugged or improperly abandoned wells, and open excavations, can

provide contaminant pathways that bypass the naturally protective clays. In general, the

unconsolidated aquifer systems of the county are most vulnerable to contamination from surface

sources where thick clay layers are lacking.

Unglaciated Southern Hills and Lowlands Aquifer System /

Dissected Till and Residuum Aquifer System

In Posey County, the Unglaciated Southern Hills and Lowlands Aquifer System and the

Dissected Till and Residuum Aquifer System are mapped as one aquifer system because they are

similar in composition and in aquifer characteristics. These systems predominantly consist of

eroded bedrock material and (in the glaciated area to the north) pre-Wisconsin till. The deposits

are relatively high in clay and silt content and fragmented rock, and are typically capped with

loess. Included in these systems are relatively thin deposits of alluvium, colluvium, and

lacustrine materials within a few of the stream valleys. In places, thin Wisconsin dune sands are

present, particularly along the edge of the Wabash River floodplain. Together the Unglaciated

Southern Hills and Lowlands Aquifer System and the Dissected Till and Residuum Aquifer

System cover about 43 percent of Posey County. The total thickness of these systems in Posey

County typically ranges from about 20 to 60 feet. In Posey County, there is little potential for

water production in these systems. All of the wells that have been reported to penetrate these

aquifer systems are completed in the underlying bedrock. Because of the low permeability of the

surface materials, these aquifer systems are not very susceptible to contamination from surface

sources.

Alluvial, Lacustrine, and Backwater Deposits Aquifer System

The Alluvial, Lacustrine, and Backwater Deposits Aquifer System is mapped adjacent to and in a

few of the valleys tributary to the Wabash and Ohio Rivers. The materials in this aquifer system

come from two major sources. One source is alluvium deposited by the streams along with

colluvium eroded from the valley walls and upland areas. The second source is glaciolacustrine

sediment, which accumulated in bodies of relatively stagnant lake water. These silts and clays

were deposited when the Wabash and Ohio River valleys were choked with coarser material

carried by glacial meltwater that effectively dammed tributary streams, creating lakes. Thick

deposits of silt and clay, sometimes called “slackwater clay,” mark the former locations of these

glacial lakes.

The Alluvial, Lacustrine, and Backwater Deposits Aquifer System is not regarded as a major

ground-water resource in this county. However, a few wells produce from this system in Mount

Vernon and the surrounding area. Well depths range from 35 to 100 feet with reported static

water levels of 10 to 30 feet below the land surface. The wells completed in the system have

been tested at rates ranging from less than 1 to 10 gallons per minute (gpm). The Alluvial,

Lacustrine, and Backwater Deposits Aquifer System in Posey County is marked by thick

deposits of soft silt and clay that have a low susceptibility to surface contamination.

Buried Valley Aquifer System

The Buried Valley Aquifer System consists of unconsolidated sediment, which was deposited in

bedrock valleys. The sediments are of variable thickness and primarily consist of lacustrine silt

and clay, glacial drift, and alluvium. Only the larger buried valleys that contain significant

water-bearing sediments have been included as mapped units of the Buried Valley Aquifer

System.

The main buried bedrock valley is mapped just south of Poseyville and extends north into

Gibson County. It cuts as deeply as about 140 feet into Pennsylvanian (McLeansboro Group)

bedrock. A tributary bedrock valley is mapped near Cynthiana and trends west.

Almost all of the wells penetrating the Buried Valley Aquifer System in Posey County were

completed in the underlying bedrock. Potential is limited in most places because of the fine-grained, commonly dirty nature of the water-bearing sand and gravel units. However, lenses of

sand and gravel (5 to10 feet thick) were reported for wells located near the town of Cynthiana

that were completed in bedrock. In addition, two wells within the Poseyville city limits formerly

produced from this system. The wells were completed in sand and gravel units 18 and 40 feet

thick. Reported capacities were 220 gpm. The wells were discontinued because of diminished

capacity and poor water quality (high iron content). The Buried Valley Aquifer System in Posey

County has a low susceptibility to surface contamination because tills and lacustrine silts and

clays generally overlie sand and gravel deposits occurring within the bedrock valleys.

Wabash Lowland Till Aquifer Subsystem

The Wabash Lowland Till Aquifer Subsystem is mapped in one area in northern Posey County.

The unconsolidated deposits consist primarily of pre-Wisconsin glacial materials and eroded

bedrock. The sediments range in thickness from about 50 feet to more than 150 feet. Loess

overlies the till across much of this system, and dune deposits consisting of mostly fine sand are

located along the western edge of the system.

In Posey County, this aquifer system is a limited resource. Potential aquifer materials within the

glacial till include sand or sand and gravel units. Only two wells producing from the system

have been reported. These two wells completed in the Wabash Lowland Till Aquifer Subsystem

have depths of 40 and 120 feet. Static water levels are 25 and 80 feet, and reported pumping

rates are 5 and 90 gpm. The Wabash Lowland Till Aquifer Subsystem typically has a low

susceptibility to surface contamination because aquifer materials are generally separated from

the surface by low-permeability layers within the system.

Wabash River and Tributaries Outwash Aquifer System /

Ohio River Outwash Aquifer System

In Posey County, the Wabash River and Tributaries Outwash Aquifer System occupies portions

of the valley of the Wabash River. The Ohio River Outwash Aquifer System occupies portions

of the main valley of the Ohio River. The watershed surface drainage divide separates the

systems.

These systems contain large volumes of sand and gravel that partially fill the main river valleys.

As the glaciers melted, the sediment contained within them was delivered to the Wabash and

Ohio Rivers in quantities too large for the streams to transport. As a result, the increased

sediment load was stored in the valleys as vertical and lateral accretionary deposits. As long as

the retreating glaciers continued to provide sediment in quantities too large for the streams to

transport, the main valleys continued to be filled. These valley-filling processes formed the most

prolific aquifer systems in the county.

The total thickness of the Wabash River and Tributaries Outwash Aquifer System / Ohio River

Outwash Aquifer System ranges from about 40 feet near the edge of the valley to 120 feet. The

saturated sand and gravel (aquifer) thickness of the systems is typically between 20 and 45 feet.

Commonly, 10 to 15 feet of silty sand and silty clay overlie the aquifer materials. However, in

some areas this layer is thin or absent.

The Wabash River and Tributaries Outwash Aquifer System / Ohio River Outwash Aquifer

System has the potential to consistently meet the needs of domestic and high-capacity water

users. Domestic wells commonly produce from 10 to 25 gpm with static water levels typically

15 to 25 feet below the land surface. There are 32 registered significant ground-water

withdrawal facilities (54 wells) using these aquifer systems in Posey County. Typical production

for high-capacity wells ranges from 500 to 800 gpm. Static water levels are generally 10 to 20

feet below the land surface. There is one significant ground-water withdrawal facility using a

radial collector well system. The reported capacity for this facility is 8000 gpm.

These aquifer systems are typically highly susceptible to contamination, because most areas lack

overlying thick layers of clay or silt. Areas within the system that are overlain by thick layers of

clay or silt are moderately susceptible to surface contamination.

Wabash River and Tributaries Outwash Aquifer Subsystem /

Ohio River Outwash Aquifer Subsystem

In Posey County, the Wabash River and Tributaries Outwash Aquifer Subsystem / Ohio River

Outwash Aquifer Subsystem is generally mapped as a transitional zone, contiguous to the

outwash systems. These systems (subsystems) are commonly mapped where the topographic

position is higher and the thickness of the aquifer is considerably less than the main outwash

aquifer systems. The saturated thickness of sand and gravel in the outwash subsystems is

typically less than 15 feet. In places, the saturated sands and gravels are overlain by a greater

thickness of silt, clay, or lacustrine deposits. Silty clay, with a thickness of 10 to 90 feet,

generally overlies the aquifer materials.

Wells are commonly completed at depths of about 40 to 100 feet in these subsystems in Posey

County. Domestic wells typically yield 5 to 20 gpm, and static water levels are generally 15 to

40 feet below the land surface. Two significant ground-water withdrawal facilities (4 wells) in

the county utilize the Wabash River and Tributaries Outwash Aquifer Subsystem / Ohio River

Outwash Aquifer Subsystem. Reported capacities range from 35 to 160 gpm. Prospects of

completing high-capacity wells in these aquifer subsystems are limited to areas with sufficient

saturated thickness.

Areas within these aquifer subsystems that lack thick overlying clay or silt deposits are highly

susceptible to contamination. However, where these systems are overlain by thick layers of clay

or silt they are only moderately susceptible to surface contamination.

Registered Significant Ground-water Withdrawal Facilities

There are 34 registered significant ground-water withdrawal facilities (total of 58 wells) using

unconsolidated aquifers in the county. Most of these facilities (32 facilities, 54 wells) utilize the

Wabash River and Tributaries Outwash Aquifer System / Ohio River Outwash Aquifer System.

Two facilities use the Wabash River and Tributaries Outwash Aquifer Subsystem / Ohio River

Outwash Aquifer Subsystem (4 wells). The facilities utilize the aquifer systems for irrigation,

public water supply, energy production, and industry. Refer to the Table for some details on the

wells and to the map for facility locations.

Map Use and Disclaimer Statement

We request that the following agency be acknowledged in products derived from this map:

Indiana Department of Natural Resources, Division of Water.

This map was compiled by staff of the Indiana Department of Natural Resources, Division of

Water using data believed to be reasonably accurate. However, a degree of error is inherent in

all maps. This product is distributed “as is” without warranties of any kind, either expressed or

implied. This map is intended for use only at the published scale.